High efficiency cascade-style heat exchanger

ABSTRACT

A heat exchanger used for wind tunnel temperature control applications. The heat exchanger is a finned tube design with each tube aligned perpendicular to the wind tunnel flow direction but with the tube bundle aligned at an oblique angle to flow direction for increased surface area. Tube fins may be aligned with the bulk flow direction. The heat exchanger is built in modules with horizontal splitter plates separating each tube bundle. The benefit of this heat exchanger design for wind tunnel applications is the combination of low pressure loss and favorable heat transfer performance in a compact design, while maintaining flow quality consistent with stringent test requirements.

This application claims benefit of and priority to U.S. ProvisionalApplication No. 61/392,980, filed Oct. 14, 2010, by Eugene A. Tennyson,et al., and is entitled to that filing date for priority. Thespecification, figures and complete disclosure of U.S. ProvisionalApplication No. 61/392,980 are incorporated herein by specific referencefor all purposes.

FIELD OF INVENTION

This invention relates to a heat exchanger used for wind tunneltemperature control.

BACKGROUND OF THE INVENTION

Wind tunnels help engineers simulate the forces acting on an objectmoving through air. To obtain useful results, the conditions in the windtunnel should closely match the conditions the object will encounter inactual service.

A fan generally drives the air flow in a wind tunnel to create the windtunnel flow stream. All of the mechanical energy supplied to the fanpropeller in a wind tunnel is converted into an increase in heat energyin the wind tunnel flow stream. Low powered wind tunnels may balancethis heat gain through various heat losses such as surface cooling andambient air exchange. High-powered wind tunnels must be cooled in orderto maintain functional testing conditions.

Since the inception of heat exchangers in wind tunnels, “fin-tube” styleor spiral wound, fluid-cooled radiator type heat exchangers have beenused. Such heat exchangers transfer the heat energy from the flow streamto a coolant. The heated coolant is then pumped out of the heatexchanger, cooled by external means such as a cooling tower, and thenrecirculated to the heat exchanger. This “fin-tube” type of heatexchanger consists of coolant-carrying tubes that cross back and forthacross the air flow passage. These tubes include attached fins thatprovide increased surface area for improved heat transfer between theflow stream and coolant.

Several problems exist with this type of heat exchanger for wind tunnelapplications. For example a “fin-tube” type of heat exchanger presents alarge resistance to the flow stream. The resistance increases the powerneeded to operate the wind tunnel at a specified wind velocity, which inturn increases the temperature of the flow even more. To reduce thisresistance, many wind tunnels increase the size of the heat exchanger inthe cross-flow stream direction. This, in turn, requires the expansionof the wind tunnel duct cross-section to house the larger heatexchanger. The transition from a smaller duct upstream of the heatexchanger to the larger duct required to house the heat exchanger mayrequire the use of a wide angle diffuser, which significantly increasesthe risk of flow separation, turbulence, and angularity problems.

Still further, the flow around the cross-stream tubes in a “fin-tube”heat exchanger produces unsteady turbulent flow characteristics. Thiscauses dynamic forces on the tube. These forces may induce tubevibration due to the low natural frequency of the slender, long spancoolant tube. The cross-stream tubes also cause flow unsteadiness andincreased turbulence in the flow steam. Unless the turbulence and flowunsteadiness is allowed to decay sufficiently, these effects may degradethe quality of the experimental results. Still further, because the finsin the heat exchanger are press fit onto the tubes, the unsteady flowand resulting vibrations over time can cause the fins to separate orlose their grip on the tube. This results in a degradation of the heattransfer effectiveness of the heat exchanger.

Some early closed circuit wind tunnels were originally built without aheat exchanger or other method to control air temperature. Retrofits tothe wind tunnel often include heat exchanger cooling systems in order tomeet the more stringent modern test conditions. In order to maintain thetop speed requirements of these wind tunnels, the flow resistance of anyinstalled heat exchanger needs to be minimized. Installation locationand overall footprint are also limited in retrofits.

Accordingly, there is a need for a wind tunnel heat exchanger structurewhich minimizes flow resistance (or pressure loss) in the wind tunneland provides adequate heat exchange in a compact area, while maintainingthe flow quality at a level suitable for wind tunnel testing.

SUMMARY OF INVENTION

In various embodiments, the present invention comprises ahigh-efficiency, cascade-style heat exchanger. The heat exchangerprovides for a more compact and efficient heat exchange capability,minimizes the flow stream blockage or resistance resulting from the heatexchanger, and provides for high aerodynamic quality of the airstreamexiting the heat exchanger.

In one exemplary embodiment, the heat exchanger has finned tubes groupedin tube bundles, each bundle comprising a plurality of tubes. Each tubeis aligned perpendicularly (i.e., across the wind tunnel) to the windtunnel flow direction but with the tube bundle aligned at an obliqueangle to the flow direction for increased surface area. In theembodiment shown, tube fins are aligned with the bulk flow directionfor, minimum airside pressure loss.

In one embodiment, the heat exchanger may be built in modules withhorizontal splitter plates separating each tube bundle. The splitterplates serve as additional flow conditioning control to reduce both flowangularity and flow non-uniformity within the wind tunnel airstream,consistent with the high level of aerodynamic flow quality required forwind tunnel testing. The tube bundles are angled relative to the bulkflow stream direction for improved heat transfer with greater surfacearea within a compact location. The angle of the tubes to the bulk flowstream, the number of modules, the height, width and length of the heatexchanger, and the number of tubes in a bundle may vary. In oneembodiment, the tube bundles are set at a 30-degree angle to the airflow, with six total modules.

The tubes are hollow and capable of transporting a coolant or fluid. Inone embodiment, the tubes are arranged in a cross-flow configuration(i.e., perpendicular) to the air flow. A fluid inlet may be located atone end or both ends of a tube bundle, with corresponding fluid outletsor drains also located at one end or both ends of the bundle.

Tubes and tube bundles are captured in a framework to create modules.The face area of these modules are angled with regard to the directionof air flow such that the face area of the overall exposed tube surfaceis larger than it would be for a planer face area. As a cold heattransfer fluid travels inside the tubes, fins attached to the tubes arecooled by conduction.

Multiple fins are attached to the tubes by mechanical extrusion (orseparately formed and attached) and can be arranged with the surfacearea aligned with or perpendicular to the air flow. The heat transferfrom the tubes to the fins cools the fins, which in turn providesconvective cooling to the airflow passing over the fins.

The heat exchanger modules are installed within a structural supportframe. This frame maintains the plane of the tube face area in theangled position thus increasing the overall face area of the heatexchanger, with respect to the flow direction of the air stream.Structural horizontal splitter plates are aligned parallel to theairflow between the heat exchanger tube bundles. The splitter platescreate an aerodynamic boundary, thereby controlling the air flow pathinto the sloped heat exchanger face.

The air flow will follow the path of least resistance and assume a pitchangle (i.e. non-horizontal flow direction) as it travels through thetube module. The pitch angle could be upward or downward depending on(a) whether the leading edge of the modules is angled so as to be aboveor below the direction of the incoming air flow and (b) the geometricconfiguration of the tubes within each module. As the airflow exits theheat exchanger tubes, the aft end of the horizontal splitter plateslocated above and below each module encourages the uniformredistribution of the airflow and a return to the incoming horizontalair flow direction. The wakes of the thin splitter plates decaynaturally downstream, returning the airflow to a uniform distributionacross the duct of the wind tunnel near the exit of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the fin-tube heat exchanger with sixmodules of tube bundles, with flow direction as shown.

FIG. 2 is an end elevation view of the heat exchanger assembly, showingslight upward flow angularity on the downstream side as the flow passesthrough the heat exchanger.

FIG. 3 is an enlarged end view of a tube bundle module with tubes endingbefore reaching the lower splitter plate.

FIG. 4 is an isometric view of a single module with support brackets,and tube bundles set at a 30-degree angle to the incoming flow.

FIG. 5 is an isometric view of a heat exchanger module showing acut-away view of tubes with intermediate supports.

FIG. 6 is an isometric view of the fin-tube heat exchanger with sixmodules of tube bundles installed within the wind tunnel walls, withflow direction as shown.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention comprises a high-efficiency, cascade-style heatexchanger. The heat exchanger provides for a more compact and efficientheat exchange capability, minimizes the flow stream blockage orresistance resulting from the heat exchanger, and provides for highaerodynamic quality of the airstream exiting the heat exchanger.

In one exemplary embodiment, as shown in FIGS. 1-3, a heat exchanger 4has tubes 14 grouped in tube bundles 12, each bundle comprising aplurality of tubes. Each tube is aligned perpendicularly (i.e., acrossthe wind tunnel) to the wind tunnel flow direction 2 but with the tubebundle 12 aligned at an oblique angle to the flow direction forincreased surface area (in the embodiment shown, a forward cant angle).Tubes have or are attached to fins 16. In the embodiment shown, tubefins 16 are aligned with the bulk flow direction 2, consistent withminimum airside pressure losses.

In one embodiment, the heat exchanger may be built in modules 20 withhorizontal splitter plates 22 separating each tube bundle. The splitterplates 22 serve as additional flow conditioning control to reduce flowangularity. In the embodiment shown in FIGS. 1 and 2, six modules 20 arestacked in a frame to form the heat exchanger.

The tube bundles 12 are angled relative to the bulk flow streamdirection 2 for improved heat transfer with greater surface area withina compact location. The angle of the tubes to the bulk flow stream, thenumber of modules, the height, width and length of the heat exchanger,and the number of tubes in a bundle may vary. In the embodiment shown inFIGS. 1-3, the tube bundles are set at a 30-degree angle to the airflow, with six total modules.

The tubes 14 are hollow and capable of transporting a coolant or fluid.In one embodiment, the tubes are arranged in a cross-flow configuration(i.e., perpendicular) to the air flow. A fluid inlet 24 may be locatedat one end or both ends of a tube bundle, with corresponding fluidoutlets or drains 26 also located at one end or both ends of the bundle.

Tubes and tube bundles are captured in a framework to create modules.The face area of these modules are angled with regard to the directionof air flow such that the face area of the overall exposed tube surfaceis larger than it would be for a planer face area. As a cold heattransfer fluid travels inside the tubes, fins 16 attached to the tubesare cooled by conduction.

Multiple fins are attached to the tubes by mechanical extrusion (orseparately formed and attached) and can be arranged with the surfacearea aligned with or perpendicular to the air flow. Alignment with airflow provides less impedance to air flow. The heat transfer from thetubes to the fins cools the fins, which in turn provides convectivecooling to the airflow passing over the fins.

The heat exchanger modules are installed within a structural supportframe 30. This frame maintains the plane of the tube face area in theangled position thus increasing the overall face area of the heatexchanger, with respect to the flow direction of the air stream.Structural horizontal splitter plates 22 are aligned parallel to theairflow between the heat exchanger tube bundles. The splitter platescreate an aerodynamic boundary, thereby controlling the air flow pathinto the sloped heat exchanger face.

The air flow will follow the path of least resistance and assume a pitchangle (i.e., non-horizontal flow direction) as it travels through thetube module. The pitch angle could be upward or downward depending on(a) whether the leading edge of the modules is angled so as to be aboveor below the direction of the incoming air flow and (b) the geometricconfiguration of the tubes within each module. As the airflow exits theheat exchanger tubes, the aft end of the horizontal splitter plateslocated above and below each module encourages the uniformredistribution of the airflow and a return to the incoming horizontalair flow direction. The wakes of the thin splitter plates decaynaturally downstream, returning the airflow to a uniform distributionacross the duct of the wind tunnel near the exit of the heat exchanger.

Thus, it should be understood that the embodiments and examplesdescribed herein have been chosen and described in order to bestillustrate the principles of the invention and its practicalapplications to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited for particular uses contemplated. Eventhough specific embodiments of this invention have been described, theyare not to be taken as exhaustive. There are several variations thatwill be apparent to those skilled in the art.

1. A heat exchanger for use in a windtunnel with a direction of bulk airflow, comprising: a frame for mounting one or more tube modules; one ormore tube modules, each tube module comprising a tube bundle with afront face and one or more fins, each tube bundle comprising a pluralityof parallel hollow tubes aligned across the direction of bulk air flow,wherein a coolant or fluid flows through the hollow tubes, and furtherwherein said fins are attached to the tubes; wherein the front face ofeach tube module is angled with respect to the direction of bulk airflow.
 2. The heat exchanger of claim 1, wherein the front face of eachtube module is angled at an oblique angle with respect to the directionof bulk air flow.
 3. The heat exchanger of claim 2, wherein the obliqueangle is 30 degrees.
 4. The heat exchanger of claim 1, wherein multiplemodules are stacked vertically in the frame.
 5. The heat exchanger ofclaim 1, wherein six tube modules are stacked vertically in the frame.6. The heat exchanger of claim 4, further comprising a splitter platelocated between adjacent tube modules, each said splitter plate alignedparallel to the direction of bulk air flow.
 7. The heat exchanger ofclaim 1, each tube module further comprising one or more fluid inletsand one or more fluid outlets.
 8. The heat exchanger of claim 1, whereinheat transfer from said tubes to said fins cools the fins, whichprovides convective cooling to the air flow passing over the fins. 9.The heat exchanger of claim 1, wherein the fins are aligned with thedirection of bulk air flow.